make doc broken

[kaeluka]

When running make doc, I get:

    Exception when trying to run compile-time code:
      ENCORE_BUNDLES: getEnv: does not exist (no environment variable)
    Code: (=<<) ((.) stringE init) (($) runIO getEnv "ENCORE_BUNDLES")
    In the splice:
      $((stringE . init)
        =<< (runIO $ System.Environment.getEnv "ENCORE_BUNDLES"))
make: *** [doc] Error 1```

[supercooldave]

This can probably be fixed by adding the code for setting the environment variable ENCORE_BUNDLES from the main makefile to the doc makefile.

I can have a look at it.

[kaeluka]

@supercooldave I have already submitted a fix: #133